let s = Symbol('hello')
let s2 = Symbol.for('world')
//检测s的symbol值是否在全局注册表注册过,返回key或者undefined
let result = Symbol.keyFor(s)
let result2 = Symbol.keyFor(s2)
console.log(result, result2);
//通过注册的s2值,找到sy值的key,再通过key找到s2值
let key = Symbol.keyFor(s2)
let result3 = Symbol.for(key)
console.log(result3 == s2);